The present invention relates generally to a load-tap changer position sensor. More particularly, the present invention is directed to a load-tap changer position sensor that uses magnetic reed switches to determine tap changer position and provides a useful and scaled analog output.
High voltage transformers are commonly used in power distribution networks. A tap changer is a device used in a transformer for regulation of the transformer output voltage within preset limits. Typically, the voltage regulation can be achieved by changing the ratio of the transformer by altering the number of turns in one winding of the transformer. An on-load tap position changer makes changes in tap position when the transformer is energized whereas as an off-load tap position changer makes changes in tap position when the transformer is de-energized. An on-load tap changer position sensor determines the position of the on-load tap changer so that various calculations associated with tap position can be performed with accurate tap position information.
On-load tap position sensors are typically coupled to the drive mechanism of the load tap changer and provide either an analog or digital output corresponding to the detected tap position. Generally, conventional on-load tap position sensors fall into one of three categories: resistor board position sensors, selsen indicators, or optical encoders.
Resistor board position sensors are the most common type of tap position sensor. These sensors use a resistor board that is mechanically coupled to the load tap changer drive mechanism. As the load tap changer is moved from tap to tap, a contact is moved from one contact to the next thereby changing the total resistance of the resistor circuit and providing a corresponding output. While generally accurate, the output of the resistor circuit can be significantly skewed if a copper oxide film develops on the surface of the resistor board.
Selsen indicators have a selsen motor which is directly coupled to the drive mechanism of the tap changer. The selsen motor includes a stator that receives an AC input voltage and a rotor having three conductors that are wired to a microprocessor. The phase angle of the AC signals from the three conductors provides tap changer position information to a microprocessor that decodes the signals to determine tap changer position. Selsen sensors offer the advantage of determining tap changer position in a non-contact manner, but disadvantageously require a microprocessor to decode the signals.
Another type of load tap changer position sensor is an optical encoder. Similar to selsen indicators, however, an optical encoder requires a processor to decode the output of the optical encoder.